This invention relates to braiding, and more particularly to high-speed single-yarn braiding.
The verb braid originally meant to intertwine or interlace three (or more) strands into a flat band or ribbon by repeatedly diagonally crossing a first strand and then a second strand alternately and under a central strand and then under the opposite strand. While such a meaning is still correct, the term braid today also encompasses the more or less tubular shaped article formed by feeding a plurality of strands off of multiple carriers and braiding the strands circumferentially around a mandrel at a controlled angle. In machine braiding the mandrel is fed through the center of the machine at a uniform rate with the machine operating like a maypole with the carriers working in counter rotating pairs with respect to the mandrel to accomplish the over and under braiding sequence. Usually, although not invariably, the mandrel is removed after formation of the braid.
Braids are conventionally produced on braiding machines as flat band or tubular constructions for a variety of purposes, including thermal and electrical insulation. For some end uses the resulting braids receive an overcoating to enhance their physical and electrical properties. Overcoating materials are usually curable organic liquids, such as varnishes, polyvinyl chloride polymers, acrylic polymers, silicone polymers, or urethane polymers, and are applied to the formed braid, which braid may of necessity be pretreated with an adhesion-promoting primer.
Yarns are of two basic types; staple filament and continuous filament. Wool and cotton fibers, as well as man-made organic or inorganic fibers if cut to a short defined length, are examples of the staple type. Individual fibers of typically 8 to 15 inches in length form the staple filament which must be spun (i.e. plied and twisted) together to form a yarn or cord. Silk fibers and extruded man-made, organic or inorganic fibers are examples of continuous filaments. Each individual fiber is of such length that it extends substantially throughout the length of any continuous filament strand.
Yarns for braiding can be of either the staple filament type or the continuous filament type depending upon the specific product application. Mechanical braiding, especially at modern high speeds, currently on the order of 420 yarn interlacings i.e. "picks" per minute, tends to result in the formation of individual fibrils (called whiskers) which project laterally from the yarns. Fibrillation is due, for example, to the frictional, tensional and shear stresses encountered by the yarns as they are machined. Fibrillation is thus predictably more prevalent in staple filament yarns, in lighter weight yarns and for more brittle materials, especially inorganic materials.
For many applications, fibrillation is not objectionable and might even be encouraged as preferable, but that is not the case for braids which are to be overcoated. Broken filament ends or staple fibers which have become untwisted, project laterally from the yarns and interfere with the overcoating process and thereby tend to detract from the aesthetic, physical and electrical properties of the overcoated braid. Continuous filament yarns are accordingly clearly preferable to staple filament yarns for overcoatable braids, especially braids of inorganic materials.
Solid waxy substances have long been used to unify yarns and to reduce frictional breakage. Wet lubrication with, for example, soap-solutions or hydrocarbon and other oils, as a means of softening and lubricating natural fibers and man-made organic fibers is also well known. Inorganic yarn materials such as glass, being relatively impervious, are usually given a solid, starch-oil based size coating during their formation and assembly into yarns. Manufacturers typically spray a size formulation solution onto the extruded filaments which dries to a finely divided, powdery solid and which improves subsequent handling and fabrication operations, said size formulation being starch-based and usually containing solid lubricants such as silanes or hydrocarbon waxes.
For applications that do not require overcoating, sized, continuous filament, inorganic single-yarns can be braided, but it has been a longstanding problem in producing overcoatable braids of inorganic yarn materials that the level of filament breakage which occurs at modern braiding speeds is sufficient to significantly and intolerably detract from the properties of the coated braid as aforesaid. This problem is especially severe with light to medium weight yarns (less than 600 tex). It should be noted however, that it is possible to braid sized single-yarns and obtain an overcoatable surface, if the braiding machine is operated at very low speeds on the order of 100 yarn interlacings per minute; speeds which are high uneconomical in today's marketplace.
Variations in size formulations and coating weights have failed to provide satisfactorily overcoatable braid surfaces at pragmatically high machine speeds. As a result, it has been the general practice in the commercial overcoatable braiding art to braid strands consisting of twisted-pairs of continuous, multi-filament, inorganic yarns, such as glass, to control breakage and fibrillation. Such twisted-pair-yarns are considerably more expensive and provide a heavier-weight braid than single-yarns, but are none-the-less more economical than production at low speeds.
The greater expense of twisted-pair-yarn is largely due to the separate twisting and plying operations on specialized machinery required to obtain the yarn. If a balanced yarn (i.e. one which will not unravel) is desired, the operations are more complex and hence even costlier. In any event, economy of material consumption is unfavorable compared to single-yarns due to the loss of length upon twisting and to the fact that the width to depth ratio is much less for twisted-pair-yarns, which requires that a greater density of strands be used to obtain an equivalent braiding surface for overcoating purposes.